Thermoelectric conversion material, thermoelectric conversion element, thermoelectric conversion module and optical sensor

ABSTRACT

A thermoelectric conversion material contains a matrix composed of a semiconductor and nanoparticles disposed in the matrix, and the nanoparticles have a lattice constant distribution Δd/d of 0.0055 or more.

TECHNICAL FIELD

The present disclosure relates to a thermoelectric conversion material,a thermoelectric conversion element, a thermoelectric conversion module,and an optical sensor. The present application claims priority toJapanese Application No. 2017-112986 filed Jun. 7, 2017, the entirecontents of which are incorporated herein by reference.

BACKGROUND ART

In recent years, renewable energy has gathered much attention as theclean energy that replaces fossil fuels such as petroleum. Examples ofthe renewable energy include not only power generation using solarenergy, hydraulic energy, and wind energy, but also power generation bythermoelectric conversion using temperature differences. Inthermoelectric conversion, since heat is directedly converted toelectricity, no superfluous waste is generated during conversion. Inaddition, thermoelectric conversion is characteristic in that themaintenance of the facility is simple since driving units such as motorsare not required. There are also optical sensors, such as infraredsensors, that utilize thermoelectric conversion.

The efficiency η of converting the temperature difference (thermalenergy) to electrical energy using a material for performingthermoelectric conversion (thermoelectric conversion material) is givenby formula (1) below:

η=ΔT/T _(h)·(M−1)/(M+T _(c) /T _(h))  (1)

where η represents the conversion efficiency, ΔT=T_(h)−T_(c), T_(h)represents the high-temperature-side temperature, T_(c) represents thelow-temperature-side temperature, M=(1+ZT)^(1/2), ZT=α²ST/κ, ZTrepresents the dimensionless figure of merit, α represents the Seebeckcoefficient, S represents the electrical conductivity, and κ representsthe thermal conductivity. As such, the conversion efficiency is amonotonically increasing function of ZT.In developing the thermoelectric conversion material, it is critical toincrease ZT.

There has been reported a technique of forming silicon germanium (SiGe)nanoparticles by performing annealing after Si, Ge, and Au serving assemiconductor materials are layered (for example, refer to NPL 1).

CITATION LIST Non Patent Literature

NPL 1: Japanese Journal of Applied Physics, 50 (2011) 041301

SUMMARY OF INVENTION

A thermoelectric conversion material according to the present disclosurecontains a matrix consisting of a semiconductor, and nanoparticlesdisposed in the matrix. The lattice constant distribution Δd/d of thenanoparticles is 0.0055 or more.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart illustrating representative steps of a method forproducing a thermoelectric conversion material according to embodiment1.

FIG. 2 is a schematic diagram of a cross-section of a part of amultilayer body, which is a thermoelectric conversion material in astate in which raw material elements are layered.

FIG. 3 is a schematic cross-sectional view of the thermoelectricconversion material according to embodiment 1.

FIG. 4 is a graph indicating one example of a peak in an X-raydiffraction signal.

FIG. 5 is a graph indicating one example of an X-ray diffraction signal.

FIG. 6 is a graph indicating results of analysis using a Williamson-Hallplot.

FIG. 7 is a diagram indicating the relationship between the latticeconstant distribution Δd/d and the thermal conductivity κ.

FIG. 8 is a schematic diagram illustrating a structure of a it-typethermoelectric conversion element (power generating element), which is athermoelectric conversion element of this embodiment.

FIG. 9 is a diagram illustrating one example of a structure of a powergenerating module.

FIG. 10 is a diagram illustrating one example of a structure of aninfrared sensor.

DESCRIPTION OF EMBODIMENTS

From the viewpoint of increasing ZT, decreasing the thermal conductivityκ in formula (1) above is considered. Here, having nanoparticles in thematrix constituting the thermoelectric conversion material can enhancephonon scattering by the nanoparticles, and, thus, the thermalconductivity can be decreased.

Recently, thermoelectric conversion materials have been required tofurther decrease thermal conductivity from the viewpoint of furtherimproving the thermoelectric conversion efficiency. Such a requirementcannot be met by the technique disclosed in NPL 1 described above.

Thus, one of the objects of the present disclosure is to provide athermoelectric conversion material with improved thermoelectricconversion efficiency.

DESCRIPTION OF EMBODIMENTS OF PRESENT DISCLOSURE

First, embodiments of the present disclosure are listed and described. Athermoelectric conversion material according to the present disclosurecontains a matrix composed of a semiconductor and nanoparticles disposedin the matrix, and the nanoparticles have a lattice constantdistribution Δd/d of 0.0055 or more.

The thermoelectric conversion material contains a matrix composed of asemiconductor. A semiconductor has a larger bandgap than conductivematerials and thus can increase the Seebeck coefficient and thedimensionless figure of merit ZT. Furthermore, since the thermoelectricconversion material contains nanoparticles disposed in the matrix, etc.,phonon scattering can be enhanced. Thus, the thermal conductivity can bedecreased, and the dimensionless figure of merit ZT can be increased.

Here, regarding the nanoparticles disposed in the matrix of thethermoelectric conversion material, the inventors have conceived offurther decreasing the thermal conductivity by increasing variation innanoparticle composition and crystal strain, in other words, variationin lattice constant of the nanoparticles. This is based on the idea thatpresence of nanoparticles with various lattice constants promotesscattering of various phonons having different frequencies. In thepresent disclosure, the thermal conductivity can be sufficientlydecreased by setting the lattice constant distribution Δd/d of thenanoparticles to 0.0055 or more.

Thus, such a thermoelectric conversion material can sufficientlyincrease the dimensionless figure of merit ZT, and can improve thethermoelectric conversion efficiency.

The matrix may be configured to be amorphous. In this manner, thethermal conductivity of the matrix, which constitutes the thermoelectricconversion material and in which nanoparticles are disposed, can bedecreased. Thus, the dimensionless figure of merit ZT can be increased,and the thermoelectric conversion efficiency can be further improved.

The lattice constant distribution Δd/d of the nanoparticles may be 0.04or less. In this range, it is easier to form the thermoelectricconversion material of the present disclosure.

The nanoparticles may have a particle diameter of 20 nm or less. In thismanner, the Seebeck coefficient can be increased, and, thus, thedimensionless figure of merit ZT can be increased. Thus, thermoelectricconversion efficiency can be further improved.

Alternatively, the semiconductor may be a SiGe-based material containingSi and Ge, an AlMnSi-based material containing Al, Mn, and Si, or aBiTe-based material containing Bi and Te. Such a base material for thesemiconductor is preferable in the thermoelectric conversion material ofthe present disclosure.

The thermoelectric conversion material may contain, as an additiveelement, at least one of Au, Cu, Al, B, Ni, and Fe. Such an additiveelement is preferable as an additive element that causes precipitationof nanoparticles in the matrix in the thermoelectric conversion materialof the present disclosure.

A thermoelectric conversion element of the present disclosure includes athermoelectric conversion material unit, a first electrode disposed tobe in contact with the thermoelectric conversion material unit, and asecond electrode disposed to be in contact with the thermoelectricconversion material unit but apart from the first electrode. Thethermoelectric conversion material unit is composed of thethermoelectric conversion material of the present disclosure describedabove, in which the component composition is adjusted to set theconductivity type to p-type or n-type.

In the thermoelectric conversion element of the present disclosure, thethermoelectric conversion material unit is composed of theaforementioned thermoelectric conversion material having excellentthermoelectric conversion properties in which the component compositionis adjusted to set the conductivity type to p-type or n-type. Thus,according to the thermoelectric conversion element of the presentdisclosure, a thermoelectric conversion element having excellentconversion efficiency can be provided.

A thermoelectric conversion module of the present disclosure includesmultiple thermoelectric conversion elements described above. Accordingto the thermoelectric conversion module of the present application,since multiple thermoelectric conversion elements of the presentapplication having excellent thermoelectric conversion efficiency areincluded, a thermoelectric conversion module having excellentthermoelectric conversion efficiency can be obtained.

An optical sensor of the present disclosure includes the thermoelectricconversion module described above. According to the optical sensor ofthe present disclosure, the aforementioned thermoelectric conversionmaterial having a sufficiently low thermal conductivity value isemployed. Thus, the optical sensor of the present disclosure can exhibithigh sensitivity.

DETAILED DESCRIPTION OF EMBODIMENTS OF PRESENT DISCLOSURE

Next, one embodiment of the thermoelectric conversion material of thepresent disclosure is described with reference to the drawings. In thedrawings described below, the same or corresponding parts are denoted bythe same reference signs, and the descriptions therefor are notrepeated.

Embodiment 1

First, a method for producing a thermoelectric conversion materialaccording to embodiment 1 is briefly described. FIG. 1 is a flowchartillustrating representative steps of a method for producing athermoelectric conversion material according to embodiment 1. Referringto FIG. 1, a material for a base substrate, such as a sapphiresubstrate, is prepared (step S11 in FIG. 1, hereinafter “step” isomitted).

Next, raw material elements constituting the thermoelectric conversionmaterial are layered on the sapphire substrate. In this case, forexample, a molecular beam epitaxy (MBE) method is employed tosequentially layer the raw material elements (S12). Specifically, alayer of amorphous silicon (Si) among the raw material elements isformed on the sapphire substrate at a room temperature. Next, a layer ofamorphous germanium (Ge), which is another raw material element, isformed on the formed amorphous Si layer. Subsequently, a layer of gold(Au) is formed on the amorphous Ge layer, and a layer of amorphous Ge isagain formed on the Au layer. Specifically, the thicknesses of therespective layers are selected to be Ge: 1.8 nm (nanometers), Au: 0.1nm, and Si: 1 nm, for example. In this case, Au is the additive element.

FIG. 2 is a schematic diagram of a cross-section of a part of amultilayer body 11, which is a thermoelectric conversion material in astate in which raw material elements are layered. FIG. 2 is across-sectional view obtained by cutting the multilayer body 11 in thethickness direction. Also referring to FIG. 2, an amorphous Si layer 13,an amorphous Ge layer 14, an Au layer 15, and another amorphous Ge layer16 are formed on the sapphire substrate 12. A multilayer unit 17constituted by the amorphous Si layer 13, the amorphous Ge layer 14, theAu layer 15, and another amorphous Ge layer 16 is repeatedly formed soas to layer the raw material elements. Layering is repeated until thetotal thickness reaches, for example, about 220 nm, and, as a result,the multilayer body 11 is formed. When the composition of the multilayerbody 11 was measured by SEM-EDX (energy dispersive X-ray spectroscopy),the Au content was 3 at %.

Next, the multilayer body 11 obtained as such is heated (S13). In thiscase, the temperature is increased from room temperature to 650° C. at arate of 10° C./min. In other words, an annealing treatment is performedby increasing the temperature relatively moderately. Nanoparticleshaving a particle diameter of about 6 nm are formed by precipitation,and thus a thermoelectric conversion material of embodiment 1 in whichthe matrix is amorphous and nanoparticles are disposed in the matrix isobtained.

FIG. 3 is a schematic cross-sectional view of a thermoelectricconversion material 21 according to embodiment 1

FIG. 3 is a cross-sectional view obtained by cutting the thermoelectricconversion material 21 in the thickness direction. Also referring toFIG. 3, the thermoelectric conversion material 21 of embodiment 1 isincludes a matrix 22 composed of a semiconductor and mainly constitutedby amorphous Si, amorphous Ge, and amorphous SiGe formed on the sapphiresubstrate 12, and nanoparticles 23 disposed in the matrix 22. In thiscase, the thermoelectric conversion material 21 is composed of aSiGe-based material containing Si and Ge as the base material for thesemiconductor. The nanoparticles 23 are formed by precipitation from Aucrystal nuclei, and are disposed in the matrix 22. The nanoparticles 23are in a microcrystalline state and are dispersed in the matrix 22. Oneexample of the specific structure of the nanoparticles 23 is a structurein which the component composition of the center region of eachnanoparticle 23 is mainly composed of SiGe. As described above, thenanoparticles 23 have a particle diameter of, for example, about 6 nm.

Here, the lattice constant distribution Δd/d of the nanoparticles 23 isset to 0.0055 or more. The lattice constant distribution Δd/d isobtained by performing X-ray diffraction on the obtained thermoelectricconversion material 21 and analyzing the diffraction results using theWilliamson-Hall plot.

Next, the definition of the lattice constant distribution Δd/d isdescribed. FIG. 4 is a graph indicating one example of a peak in anX-ray diffraction signal. In FIG. 4, the vertical axis indicates theintensity (a.u.) of the X-ray diffraction signal, and the horizontalaxis indicates 2θ(°) where θ represents the diffraction angle.

Referring now to FIG. 4, first, assuming the position of a peak 31 inthe X-ray diffraction signal to be at 2θ(peak), the lattice constant dat the position of the peak 31 is determined. Here, since therelationship between the lattice constant and the diffraction angle isexpressed by the relationship, 2d sin θ=nλ (d: lattice constant, λ:wavelength), this relationship is used. Next, the high-angle-side valueat the signal position constituting the half value of the peak 31 isdetermined and is defined as 2θ(H). Then, 2θ(H) is used to determine thehigh-angle-side lattice constant d(H). In addition, the low-angle-sidevalue at the signal position constituting the half value of the peak 31is determined and is defined as 2θ(L). Then, 2θ(L) is used to determinethe low-angle-side lattice constant d(L). In the lattice constantdistribution Δd/d, Δd is determined as d(H)−d(L). Lastly, the latticeconstant distribution Δd/d is determined as (d(H)−d (L))/d. Thus, thelattice constant distribution Δd/d is defined. The lattice constantdistribution M/d indicated here is based on the shape of the peak 31attributable to the crystals of a predetermined particle diameter asillustrated in FIG. 4.

In an actual system, since particle diameters vary, two or more X-raydiffraction signals must be analyzed, and the analysis must be performedby using a Williamson-Hall plot with which the lattice constantdistribution and the particle diameter can be separated. The analysisexample therefor is described below.

FIG. 5 is a graph indicating one example of an X-ray diffraction signal.In FIG. 5, the vertical axis indicates the intensity (a.u.) of the X-raydiffraction signal, and the horizontal axis indicates 2θ(°) where θrepresents the diffraction angle. In FIG. 5, 2θ is set to 20° to 70°.Moreover, a solid line 32 indicates the case in which a radiated X-rayis used as the X-ray in the X-ray diffraction, and a broken line 33indicates the case in which Cu K-α radiation is used as the X-ray in theX-ray diffraction. Moreover, FIG. 5 indicates the case in which thethickness of the Au layer 15, in other words, Au, is set to 0.35 nmbefore the heat treatment illustrated in FIG. 2 described above and thenthe heating step is completed, in other words, the state in which thenanoparticles 23 are precipitated. Note that the X-ray diffraction usingthe radiated X-ray is performed in a large-scale radiation facility,SPring-8. Moreover, in the graph of FIG. 5, the deviation between thesolid line 32 and the broken line 33 is attributable to the differencein the type of X-ray used in the X-ray diffraction or the like.

The measurement conditions of the measurement using synchrotronradiation and SPring-8 are as follows: beamline: SPring-8 BL16XU,spectroscope/mirror: Si(111)/Rh coat mirror 3.5 mrad, photon energy: 18keV (0.689 Å), detector: two-dimensional detector PILATUS 100K, cameralength: 200 mm, slit width: 50 mm (H)×500 mm (V), incident angle: 0.5°,detector center angle: 19°, and exposure time: 3 seconds. Themeasurement conditions of the measurement using Cu K-α radiation areXpert (45 kV, 40 mA) produced by Panalytical. When a general purposeX-ray diffraction facility is used, the influence of mixing of the CuK-β radiation on the high angle side becomes significant, and thus, themeasurement is preferably conducted by using SPring-8 from the viewpointof the measurement accuracy.

Referring to FIG. 5, for example, a peak 34 a indicated by the solidline 32 and the broken line 33 at about 28° indicates SiGe. As such, inthe diffraction signal, multiple peaks 34 a, 34 b, 34 c, 34 d, 34 e, 34f, 34 g, 34 h, 34 i, 34 j, 34 k, 341, and 34 m appear.

Regarding the X-ray diffraction results obtained as such, the inventorshave focused on the fact that the information regarding the particlediameters of the nanoparticles and the lattice constant distribution ofthe nanoparticles is contained in the information regarding the halfvalue widths of the peaks and the positions of the peaks in thediffraction signal. Thus, analysis is performed using theWilliamson-Hall plot to separate the element regarding the particlediameters of the nanoparticles 23 and the element regarding the latticeconstant distribution of the nanoparticles 23.

FIG. 6 is a graph indicating results of analysis through aWilliamson-Hall plot. In FIG. 6, the vertical axis indicates the valueof β cos θ/λ, and the horizontal axis indicates the value of sin θ/λ.Four solid triangles, four squares, and four rhombuses in FIG. 6respectively indicate the same sample. Here, β represents the half valuewidth, θ represents the diffraction angle, and λ represents thewavelength of the X-ray. In FIG. 6, these values are plotted for fourdiffraction signals. For each of the samples, the plotted points wereconnected by straight lines 36 a, 36 b, and 36 c. The straight line 36 aindicates the sample in which the thickness of Au is set to 0.35 nm, thestraight line 36 b indicates the sample in which the thickness of Au isset to 0.17 nm, and the straight line 36 c indicates the sample in whichthe thickness of Au is set to 0.10 nm. For these straight lines 36 a to36 c, the relationship β cos θ/λ=2η sin θ/λ+1/ε is satisfied, where ηrepresents the lattice constant distribution, 1/ε represents theparticle diameter, and β cos θ and sin θ represent variables. In otherwords, the slope 2η of the straight line indicates the lattice constantdistribution Δd/d, and the intercept 1/ε indicates the particlediameter.

The relationship between the thickness (nm) of Au, the particle diameter(nm), and the lattice constant distribution (Δd/d) derived from thestraight lines 36 a to 36 c illustrated in FIG. 6 is as follows. Thatis, when the thickness of Au is 0.35 nm, the particle diameter is 25 nm,and the lattice constant distribution Δd/d is 0.0028. When the thicknessof Au is 0.17 nm, the particle diameter is 25 nm, and the latticeconstant distribution Δd/d is 0.0055. When the thickness of Au is 0.10nm, the particle diameter is 20 nm, and the lattice constantdistribution Δd/d is 0.0065.

Here, the inventors have investigated the relationship between theobtained lattice constant distribution Δd/d and the thermal conductivityκ. FIG. 7 is a graph indicating the relationship between the latticeconstant distribution Δd/d and the thermal conductivity κ. In FIG. 7,the vertical axis indicates the thermal conductivity (W/mK), and thehorizontal axis indicates the lattice constant distribution Δd/d. Thethermal conductivity κ is measured from the samples.

Referring to FIG. 7, regarding the relationship between the thermalconductivity κ and the lattice constant distribution Δd/d, when thelattice constant distribution Δd/d takes a value of 0.0028, the thermalconductivity κ is 0.36. When the lattice constant distribution Δd/dtakes a value of 0.004, the thermal conductivity κ is 0.25. When thelattice constant distribution Δd/d takes a value of 0.0055, the thermalconductivity κ is 0.16. When the lattice constant distribution Δd/dtakes a value of 0.0065, the thermal conductivity κ is 0.16. When thelattice constant distribution Δd/d takes a value of 0.0075, the thermalconductivity κ is 0.15. When the lattice constant distribution Δd/dtakes a value of 0.0090, the thermal conductivity κ is 0.16. When thelattice constant distribution M/d takes a value of 0.010, the thermalconductivity κ is 0.15. When the lattice constant distribution Δd/dtakes a value of 0.040, the thermal conductivity κ is 0.16. Here,regarding the thermal conductivity κ, a sufficiently small value can beobtained by setting the lattice constant distribution Δd/d to a valueequal to or more than the threshold value, which is 0.0055. In otherwords, the thermal conductivity κ can be adjusted to a very small valueby adjusting the value of the lattice constant distribution Δd/d to0.0055 or more.

This is considered as follows. That is, when the variation in crystalcomposition and the composition strain of the nanoparticles 23 roughlyindicated by the lattice constant distribution Δd/d of the nanoparticles23 increase, a so-called compositional non-uniformity occurs. Thispromotes scattering of phonons having different frequencies. Presumablythus, the thermal conductivity κ can be sufficiently decreased. Thus, insuch a thermoelectric conversion material 21, the dimensionless figureof merit ZT can be sufficiently increased, and the thermoelectricconversion efficiency can be improved. Presumably, when thenanoparticles 23 are constituted by covalent interatomic bonds, therelationship between phonon scattering and the compositionalnon-uniformity with which the lattice constant distribution Δd/d takes avalue of 0.0055 or more can be applied. Moreover, the limit of thelattice constant distribution Δd/d of a covalent material is 0.04. Thelimit is the value determined from the lattice constants of Si and Ge inthe Si—Ge compound, which is a covalently bonded all-proportionalsolid-solution. It is considered that the limit of the lattice constantdistribution Δd/d is 0.04 for all other covalent crystal materials inthe present disclosure.

The value of the lattice constant distribution Δd/d is preferably 0.04or less. In this range, it is easy to form the thermoelectric conversionmaterial 21 of the present application.

In the embodiment described above, the matrix 22 of the thermoelectricconversion material 21 is amorphous. However, this is not limiting, andthe matrix 22 may be polycrystalline.

In the embodiment described above, the semiconductor is a SiGe-basedmaterial containing Si and Ge. However, this it not limiting, and thecovalent semiconductor may be an AlMnSi-based material containing Al,Mn, and Si, or a BiTe-based material containing Bi and Te. Such asemiconductor is preferable for the thermoelectric conversion materialof the present disclosure. This is because it is considered that therelationship between the compositional non-uniformity and the phononscattering found in the present disclosure can be applied as long as thenanoparticles 23 are a crystallized covalent semiconductor material.

In the embodiment described above, Au is used as the additive element.However, this is not limiting, and the additive element may contain atleast one of Au, Cu, Al, B, Ni, and Fe. These elements can serve asnuclei in the matrix composed of a semiconductor, and thus arepreferable as the additive elements for inducing precipitation of thenanoparticles 23.

The particle diameter of the nanoparticles 23 is preferably 20 nm orless. In this manner, the phonon scattering can be enhanced and thus thethermal conductivity can be decreased. Moreover, the Seebeck coefficientcan be increased, and, thus, ZT can be increased. Thus, thermoelectricconversion efficiency can be further improved. The particle diameter ofthe nanoparticles 23 is more preferably 10 nm or less and yet morepreferably 5 nm or less. In this manner, phonon scattering can befurther enhanced, and the thermal conductivity can be further decreased.

Embodiment 2

Next, a power generating element and a power generating module, whichare embodiments of the thermoelectric conversion element and thethermoelectric conversion module that use the thermoelectric conversionmaterial of the present disclosure, are described.

FIG. 8 is a schematic diagram illustrating a structure of a π-typethermoelectric conversion element (power generating element) 51, whichis a thermoelectric conversion element of this embodiment. Referring toFIG. 8, the π-type thermoelectric conversion element 51 includes ap-type thermoelectric conversion material unit 52 that serves as a firstthermoelectric conversion material unit, an n-type thermoelectricconversion material unit 53 which serves as a second thermoelectricconversion material unit, a high-temperature-side electrode 54, a firstlow-temperature-side electrode 55, a second low-temperature-sideelectrode 56, and a wire 57.

The p-type thermoelectric conversion material unit 52 is, for example,composed of the thermoelectric conversion material of embodiment 1 inwhich the component composition is adjusted to set the conductivity typeto p-type. The conductivity type of the p-type thermoelectric conversionmaterial unit 52 is set to p-type, for example, by doping thethermoelectric conversion material of embodiment 1 constituting thep-type thermoelectric conversion material unit 52 with a p-type impuritythat generates a p-type carrier (holes) that serves as a majoritycarrier.

The n-type thermoelectric conversion material unit 53 is composed of,for example, the thermoelectric conversion material of embodiment 1 inwhich the component composition is adjusted to set the conductivity typeto n-type. The conductivity type of the n-type thermoelectric conversionmaterial unit 53 is set to n-type, for example, by doping thethermoelectric conversion material of embodiment 1 constituting then-type thermoelectric conversion material unit 53 with an n-typeimpurity that generates an n-type carrier (electrons) that serves as amajority carrier.

The p-type thermoelectric conversion material unit 52 and the n-typethermoelectric conversion material unit 53 are arranged side-by-sidewith a space therebetween. The high-temperature-side electrode 54 isarranged to extend from one end portion 61 of the p-type thermoelectricconversion material unit 52 to one end portion 62 of the n-typethermoelectric conversion material unit 53. The high-temperature-sideelectrode 54 is arranged to contact both the end portion 61 of thep-type thermoelectric conversion material unit 52 and the end portion 62of the n-type thermoelectric conversion material unit 53. Thehigh-temperature-side electrode 54 is arranged to connect the endportion 61 of the p-type thermoelectric conversion material unit 52 andthe end portion 62 of the n-type thermoelectric conversion material unit53. The high-temperature-side electrode 54 is composed of, for example,a conductive material such as a metal. The high-temperature-sideelectrode 54 makes an ohmic contact with the p-type thermoelectricconversion material unit 52 and the n-type thermoelectric conversionmaterial unit 53.

The thermoelectric conversion material unit 52 or the thermoelectricconversion material unit 53 is preferably p-type or n-type; however,alternatively, one the thermoelectric conversion material units 52 and53 may be a metal conductor.

In the embodiment described above, a π-type thermoelectric conversionelement is described as one example of the thermoelectric conversionelement of the present application, but the thermoelectric conversionelement of the present application is not limited to this. Thethermoelectric conversion element of the present application may haveother structures such as an I-type (uni-leg type) thermoelectricconversion element, for example.

The first low-temperature-side electrode 55 is arranged to contactanother end portion 63 of the p-type thermoelectric conversion materialunit 52. The first low-temperature-side electrode 55 is arranged to beapart from the high-temperature-side electrode 54. The firstlow-temperature-side electrode 55 is composed of, for example, aconductive material such as a metal. The first low-temperature-sideelectrode 55 makes an ohmic contact with the p-type thermoelectricconversion material unit 52.

The second low-temperature-side electrode 56 is arranged to contactanother end portion 64 of the n-type thermoelectric conversion materialunit 53. The second low-temperature-side electrode 56 is arranged to beapart from the high-temperature-side electrode 54 and the firstlow-temperature-side electrode 55. The second low-temperature-sideelectrode 56 is composed of, for example, a conductive material such asa metal. The second low-temperature-side electrode 56 makes an ohmiccontact with the n-type thermoelectric conversion material unit 53.

The wire 57 is composed of a conductor such as a metal. The wire 57electrically connects the first low-temperature-side electrode 55 andthe second low-temperature-side electrode 56.

In the π-type thermoelectric conversion element 51, when, for example, atemperature difference is formed between the high temperature at the endportion 61 of the p-type thermoelectric conversion material unit 52 andthe end portion 62 of the n-type thermoelectric conversion material unit53 and the low temperature at the end portion 63 of the p-typethermoelectric conversion material unit 52 and the end portion 64 of then-type thermoelectric conversion material unit 53, the p-type carrier(holes) migrates in the p-type thermoelectric conversion material unit52 from the end portion 61 toward the end portion 63. During thisprocess, in the n-type thermoelectric conversion material unit 53, then-type carrier (electrons) migrates from the end portion 62 toward theend portion 64. As a result, an electric current flows in the wire 57 inthe arrow a direction. Thus, in the π-type thermoelectric conversionelement 51, power is generated by the thermoelectric conversion usingthe temperature difference. In other words, the π-type thermoelectricconversion element 51 is a power generating element.

The thermoelectric conversion material of embodiment 1 in which thevalue of ZT is increased by sufficiently decreasing the thermalconductivity is employed as the material that makes up the p-typethermoelectric conversion material unit 52 and the n-type thermoelectricconversion material unit 53. As a result, the π-type thermoelectricconversion element 51 serves as a high-efficiency power generatingelement.

Furthermore, a power generating module that serves as a thermoelectricconversion module can be obtained by electrically connecting multipleπ-type thermoelectric conversion elements 51. A power generating module65, which is a thermoelectric conversion module of this embodiment, hasa structure in which multiple π-type thermoelectric conversion elements51 are connected in series.

FIG. 9 is a diagram illustrating one example of a structure of the powergenerating module 65. Referring to FIG. 9, the power generating module65 of this embodiment includes p-type thermoelectric conversion materialunits 52, n-type thermoelectric conversion material units 53, andlow-temperature-side electrodes 55, 56 each corresponding to the firstlow-temperature-side electrode 55 and the second low-temperature-sideelectrode 56, high-temperature-side electrodes 54, alow-temperature-side insulator substrate 66, and a high-temperature-sideinsulator substrate 67. The low-temperature-side insulator substrate 66and the high-temperature-side insulator substrate 67 are composed of aceramic such as alumina. The p-type thermoelectric conversion materialunits 52 and the n-type thermoelectric conversion material units 53 arealternately arranged side-by-side. The low-temperature-side electrodes55, 56 are arranged to be in contact with the p-type thermoelectricconversion material units 52 and the n-type thermoelectric conversionmaterial units 53 as with the π-type thermoelectric conversion element51 described above. The high-temperature-side electrode 54 is arrangedto be in contact with the p-type thermoelectric conversion materialunits 52 and the n-type thermoelectric conversion material units 53 aswith the π-type thermoelectric conversion element 51 described above.Each p-type thermoelectric conversion material unit 52 is connected toan adjacent n-type thermoelectric conversion material unit 53 on oneside via a common high-temperature-side electrode 54. Each p-typethermoelectric conversion material unit 52 is connected to an adjacentn-type thermoelectric conversion material unit 53 on another sidedifferent from the aforementioned one side via commonlow-temperature-side electrode 55, 56.

As a result, all of the p-type thermoelectric conversion material units52 and the n-type thermoelectric conversion material units 53 areconnected in series.

The low-temperature-side insulator substrate 66 is disposed on a mainsurface side of the plate-shaped low-temperature-side electrode 55, 56,the main surface side being opposite to the side contacting the p-typethermoelectric conversion material units 52 and the n-typethermoelectric conversion material units 53.

One low-temperature-side insulator substrate 66 is provided for multiple(all) low-temperature-side electrodes 55, 56. The high-temperature-sideinsulator substrate 67 is disposed on a side of the plate-shapedhigh-temperature-side electrodes 54, the side being opposite to the sidecontacting the p-type thermoelectric conversion material units 52 andthe n-type thermoelectric conversion material units 53. Onehigh-temperature-side insulator substrate 67 is provided for multiple(all) high-temperature-side electrodes 54.

Wires 68 and 69 are respectively connected to the high-temperature-sideelectrodes 54 or low-temperature-side electrodes 55, 56 that contact thep-type thermoelectric conversion material units 52 or n-thermoelectricconversion material units 53 located at two ends among the seriallyconnected p-type thermoelectric conversion material units 52 and then-type thermoelectric conversion material units 53. When a temperaturedifference is formed between the high temperature on the side of thehigh-temperature-side insulator substrate 67 and the low temperature onthe side of the low-temperature-side insulator substrate 66, an electriccurrent flows in the arrow c direction through the serially connectedp-type thermoelectric conversion material units 52 and the n-typethermoelectric conversion material units 53 as in the aforementionedcase of the π-type thermoelectric conversion element 51. Thus, in thepower generating module 65, power is generated by thermoelectricconversion using the temperature difference.

Embodiment 3

Next, an infrared sensor, which is an optical sensor, is described asanother embodiment of the thermoelectric conversion element that usesthe thermoelectric conversion material of the present disclosure.

FIG. 10 is a diagram illustrating one example of a structure of aninfrared sensor 71. Referring to FIG. 10, the infrared sensor 71includes a p-type thermoelectric conversion unit 72 and an n-typethermoelectric conversion unit 73 that are arranged to be adjacent toeach other. The p-type thermoelectric conversion unit 72 and the n-typethermoelectric conversion unit 73 are formed on a substrate 74.

The infrared sensor 71 includes the substrate 74, an etching stop layer75, an n-type thermoelectric conversion material layer 76, an n⁺-typeohmic contact layer 77, an insulator layer 78, a p-type thermoelectricconversion material layer 79, an n-side ohmic contact electrode 81, ap-side ohmic contact electrode 82, a heat-absorbing pad 83, an absorber84, and a protective film 85.

The substrate 74 is composed of an insulator such as silicon dioxide. Arecess 86 is formed in the substrate 74. The etching stop layer 75 isformed to cover the surface of the substrate 74. The etching stop layer75 is composed of, for example, an insulator such as silicon nitride. Aspace is formed between the etching stop layer 75 and the recess 86 inthe substrate 74.

The n-type thermoelectric conversion material layer 76 is formed on amain surface of the etching stop layer 75, the main surface being on theopposite side of the substrate 74. The n-type thermoelectric conversionmaterial layer 76 is composed of the thermoelectric conversion materialof embodiment 1 in which the component composition is adjusted to setthe conductivity type to n-type, for example. The conductivity type ofthe n-type thermoelectric conversion material layer 76 is set to n-type,for example, by doping the thermoelectric conversion material ofembodiment 1 constituting the n-type thermoelectric conversion materiallayer 76 with an n-type impurity that generates an n-type carrier(electrons) that serves as a majority carrier. The n⁺-type ohmic contactlayer 77 is formed on a main surface of the n-type thermoelectricconversion material layer 76, the main surface being on the oppositeside of the etching stop layer 75. The n⁺-type ohmic contact layer 77is, for example, doped with an n-type impurity that generates an n-typecarrier (electrons) that serves as a majority carrier so that theconcentration of the n-type impurity is higher than in the n-typethermoelectric conversion material layer 76. As a result, theconductivity type of the n⁺-type ohmic contact layer 77 is n-type.

The n-side ohmic contact electrode 81 is disposed so as to contact acenter portion of a main surface of the n⁺-type ohmic contact layer 77,the main surface being on the opposite side of the n-type thermoelectricconversion material layer 76. The n-side ohmic contact electrode 81 iscomposed of a material that can make an ohmic contact with the n⁺-typeohmic contact layer 77, for example, a metal. The insulator layer 78composed of an insulator such as silicon dioxide is disposed on a mainsurface of the n⁺-type ohmic contact layer 77, the main surface being onthe opposite side of the n-type thermoelectric conversion material layer76. The insulator layer 78 is disposed on a main surface of the n⁺-typeohmic contact layer 77, the main surface being on the p-typethermoelectric conversion unit 72 side when viewed from the n-side ohmiccontact electrode 81.

The protective film 85 is further provided on a main surface of then⁺-type ohmic contact layer 77, the main surface being on the oppositeside of the n-type thermoelectric conversion material layer 76. Theprotective film 85 is disposed on a main surface of the n⁺-type ohmiccontact layer 77, the main surface being on the opposite side of thep-type thermoelectric conversion layer 72 when viewed from the n-sideohmic contact electrode 81. Another n-side ohmic contact electrode 81 isdisposed on a main surface of the n⁺-type ohmic contact layer 77, themain surface being on the opposite side of the n-type thermoelectricconversion material layer 76, is disposed on the opposite side of theaforementioned n-side ohmic contact electrode 81 with the protectivefilm 85 therebetween.

The p-type thermoelectric conversion material layer 79 is disposed on amain surface of the insulator layer 78, the main surface being on theopposite side of the n⁺-type ohmic contact layer 77. The p-typethermoelectric conversion material layer 79 is composed of thethermoelectric conversion material of embodiment 1 in which thecomponent composition is adjusted to set the conductivity type top-type. The conductivity type of the p-type thermoelectric conversionmaterial layer 79 is set to p-type, for example, by doping thethermoelectric conversion material of embodiment 1 constituting thep-type thermoelectric conversion material layer 79 with a p-typeimpurity that generates a p-type carrier (holes) that serves as amajority carrier.

The protective film 85 is disposed in a center portion on a main surfaceof the p-type thermoelectric conversion material layer 79, the mainsurface being on the opposite side of the insulator layer 78. A pair ofp-side ohmic contact electrodes 82 that flank the protective film 85 aredisposed on a main surface of the p-type thermoelectric conversionmaterial layer 79, the main surface being on the opposite side of theinsulator layer 78. The p-side ohmic contact electrodes 82 are composedof a material that can make an ohmic contact with the p-typethermoelectric conversion material layer 79, for example, a metal. Ofthe pair of the p-side ohmic contact electrodes 82, the p-side ohmiccontact electrode 82 on the n-type thermoelectric conversion unit 73side is connected to the n-side ohmic contact electrode 81.

The absorber 84 is disposed to cover main surfaces of the p-side ohmiccontact electrode 81 and the n-side ohmic contact electrode 82 connectedto each other, the main surfaces being on the opposite side of then⁺-type ohmic contact layer 77. The absorber 84 is composed of, forexample, titanium. The heat-absorbing pad 83 is disposed so as tocontact the top of the p-type ohmic contact electrode 81 not connectedto the n-side ohmic contact electrode 82. The heat-absorbing pad 83 isdisposed so as to contact the top of the n-type ohmic contact electrode82 not connected to the p-side ohmic contact electrode 81. An example ofthe material constituting the heat-absorbing pad 83 is gold(Au)/titanium (Ti).

When an infrared ray is applied to the infrared sensor 71, the absorber84 absorbs the energy of the infrared ray. As a result, the temperatureof the absorber 84 rises. Meanwhile, the increase in temperature of theheat-absorbing pad 83 is suppressed. Thus, a temperature difference isformed between the absorber 84 and the heat-absorbing pad 83. Thus, inthe p-type thermoelectric conversion material layer 79, the p-typecarrier (holes) migrates from the absorber 84 toward the heat-absorbingpad 83. Meanwhile, in the n-type thermoelectric conversion materiallayer 76, the n-type carrier (electrons) migrates from the absorber 84toward the heat-absorbing pad 83. Then, an electric current generated asa result of the migration of the carriers from the n-side ohmic contactelectrode 81 and the p-side ohmic contact electrode 82 is extracted andthe infrared ray is detected as a result.

The infrared sensor 71 of this embodiment employs the thermoelectricconversion material of embodiment 1, in which the value of ZT isincreased by sufficiently decreasing the thermal conductivity, as thematerial that makes up the p-type thermoelectric conversion materiallayer 79 and the n-type thermoelectric conversion material layer 76.Thus, the infrared sensor 71 exhibits high sensitivity.

The embodiments disclosed herein are exemplary in all aspects, andshould not be understood as limiting from any respect. The scope of thepresent invention is defined not by the descriptions above but by theclaims and is intended to include all alterations and modificationswithin the meaning of the claims and equivalents thereof.

REFERENCE SIGNS LIST

-   11 multilayer body-   12 sapphire substrate-   13 amorphous Si layer-   14, 16 amorphous Ge layer-   15 Au layer-   17 multilayer unit-   21 thermoelectric conversion material-   22 matrix-   23 nanoparticle-   31, 34 a, 34 b, 34 c, 34 d, 34 e, 34 f, 34 g, 34 h, 34 i, 34 j, 34    k, 341, 34 m peak-   32 solid line-   33 broken line-   36 a, 36 b, 36 c straight line-   51 π-type thermoelectric conversion element-   52 p-type thermoelectric conversion material unit-   53 n-type thermoelectric conversion material unit-   54 high-temperature-side electrode-   55 first low-temperature-side electrode (low-temperature-side    electrode)-   56 second low-temperature-side electrode (low-temperature-side    electrode)-   57, 68, 69 wire-   61, 62, 63, 64 end portion-   65 power generating module-   66 low-temperature-side insulator substrate-   67 high-temperature-side insulator substrate-   71 infrared sensor-   72 p-type thermoelectric conversion unit-   73 n-type thermoelectric conversion unit-   74 substrate-   75 etching stop layer-   76 n-type thermoelectric conversion material layer-   77 n⁺-type ohmic contact layer-   78 insulator layer-   79 p-type thermoelectric conversion material layer-   81 n-side ohmic contact electrode-   82 p-side ohmic contact electrode-   83 heat-absorbing pad-   84 absorber-   85 protective film

1: A thermoelectric conversion material comprising: a matrix consistingof a semiconductor; and nanoparticles disposed in the matrix, whereinthe nanoparticles have a lattice constant distribution Δd/d of 0.0055 ormore. 2: The thermoelectric conversion material according to claim 1,wherein the matrix is amorphous. 3: The thermoelectric conversionmaterial according to claim 1, wherein the nanoparticles have a latticeconstant distribution Δd/d of 0.04 or less. 4: The thermoelectricconversion material according to claim 1, wherein the nanoparticles havea particle diameter of 20 nm or less. 5: The thermoelectric conversionmaterial according to claim 1, wherein the semiconductor is anSiGe-based material containing Si and Ge, an AlMnSi-based materialcontaining Al, Mn, and Si, or a BiTe-based material containing Bi andTe. 6: The thermoelectric conversion material according to claim 1,containing, as an additive element, at least one of Au, Cu, Al, B, Ni,and Fe. 7: A thermoelectric conversion element comprising: athermoelectric conversion material unit; a first electrode disposed tobe in contact the thermoelectric conversion material unit; and a secondelectrode disposed to be in contact the thermoelectric conversionmaterial unit and to be away from the first electrode, wherein thethermoelectric conversion material unit consists of the thermoelectricconversion material according to claim 1, the thermoelectric conversionmaterial having a component composition adjusted to set a conductivitytype to p-type or n-type. 8: A thermoelectric conversion modulecomprising a plurality of the thermoelectric conversion elementsaccording to claim
 7. 9: An optical sensor comprising the thermoelectricconversion module according to claim
 8. 10: The thermoelectricconversion material according to claim 2, wherein the nanoparticles havea lattice constant distribution Δd/d of 0.04 or less. 11: Thethermoelectric conversion material according to claim 2, wherein thenanoparticles have a particle diameter of 20 nm or less. 12: Thethermoelectric conversion material according to claim 3, wherein thenanoparticles have a particle diameter of 20 nm or less. 13: Thethermoelectric conversion material according to claim 2, wherein thesemiconductor is an SiGe-based material containing Si and Ge, anAlMnSi-based material containing Al, Mn, and Si, or a BiTe-basedmaterial containing Bi and Te. 14: The thermoelectric conversionmaterial according to claim 3, wherein the semiconductor is anSiGe-based material containing Si and Ge, an AlMnSi-based materialcontaining Al, Mn, and Si, or a BiTe-based material containing Bi andTe. 15: The thermoelectric conversion material according to claim 4,wherein the semiconductor is an SiGe-based material containing Si andGe, an AlMnSi-based material containing Al, Mn, and Si, or a BiTe-basedmaterial containing Bi and Te. 16: The thermoelectric conversionmaterial according to claim 12, wherein the semiconductor is anSiGe-based material containing Si and Ge, an AlMnSi-based materialcontaining Al, Mn, and Si, or a BiTe-based material containing Bi andTe. 17: The thermoelectric conversion material according to claim 2,containing, as an additive element, at least one of Au, Cu, Al, B, Ni,and Fe. 18: The thermoelectric conversion material according to claim 3,containing, as an additive element, at least one of Au, Cu, Al, B, Ni,and Fe. 19: The thermoelectric conversion material according to claim 4,containing, as an additive element, at least one of Au, Cu, Al, B, Ni,and Fe. 20: The thermoelectric conversion material according to claim 5,containing, as an additive element, at least one of Au, Cu, Al, B, Ni,and Fe. 21: The thermoelectric conversion material according to claim13, containing, as an additive element, at least one of Au, Cu, Al, B,Ni, and Fe.